1. Field of Invention
The present invention relates to building materials, and is more particularly concerned with acoustical tile suitable for use in ceiling structures having good acoustical properties, good fire retardance, and being free of undesirable warping.
2. Prior Art
Acoustical tile for use in sound absorbing ceilings has found wide use in the industry. U.S. Pat. No. 1,769,519 discloses such an acoustical tile which, according to its teachings, is formed of a mixture of granulated mineral wool, fillers, certain coloring materials, if needed, and a binder, particularly one of an amylaceous nature, such as thick boiling starch. This mixture or composition is placed upon suitable trays which have been previously covered with paper, such as newsprint, and then screeded to a suitable thickness with a reciprocating edge. A pleasing surface, including elongated fissures, resembling that of travertine stone is normally obtained. Alternatively, by screeding in a different manner the surface can be made without the fissures. The trays are then placed in an oven, and dried or cured. The dried sheets, called slabs, are removed from the mold, and dressed on both faces to provide smooth surfaces, to obtain the desired thickness and to prevent warping, and are then cut into tiles of a desired size.
Previous to this invention it had been assumed by those skilled in this art that for maximum drying speed the moisture should leave from both the bottom and the top surfaces of the drying slab and that covering the bottom surface with an impervious lamina would increase the drying time. Accordingly, the tray bottoms were made of foraminous material and covered with thin, relatively unsized layers of paper so as to facilitate the passage of water out of the back surface of the tile through the paper. Drying the composition under these conditions resulted in migration of the starch to both the bottom and top surfaces where it strengthened the surface areas. During the dressing operation, the face surface of the slab was normally sanded off to obtain a pleasant smooth surface, thereby removing a portion of the face area of high starch concentration. The resulting tiles were found to warp unless the corresponding back surface area of high starch content was also sanded off. Thus, the back surface of the slabs was dressed by sanding off the paper and a portion of the hardened composition to compensate for sanding of the other (face) side. Sanding the paper back on conventional tiles is also required to pass the E-84 flame spread test, and is at least as important as avoiding warping.
Removing the back surface of the slabs in the aforenoted process is not only a time-consuming and costly operation but also results in the loss of the paper and part of the cured composition, thereby necessitating the use and curing of extra material in initially forming the slabs.
In U.S. Pat. No. 3,307,651 there is disclosed an acoustical tile of the type described having an aluminum foil backing. Such a tile has improved fire-retardance properties and acoustical properties. Additionally, the aluminum backing serves to release the finished tiles from the trays in which they are formed. Additionally, this tile exhibits excellent sound attenuation properties, that is, it prevents to a high degree sound from passing through the tile. However, this tile has exhibited a significant adverse property. When the face surface of the tile is subsequently painted with a finishing material such as paint and the material dried, this causes the tile to warp, with the face of the tile becoming concave. This effect is also known as cupping or lipping. The undesirable feature of this condition is that when any kind of grazing light strikes the face of a tile having a concave surface, the edges of the tile are accentuated, and in the more aggravated condition the edges may actually form ridges in exaggerated form or even stand out from adjacent tiles. In the case of standard tile not having an aluminum foil backing, this condition is prevented by wetting the back surface with water simultaneously with the painting and drying operation. This completely eliminates the problem. However, such tiles have inferior acoustical sound attenuating properties.